A memory is said to be a sequential access memory when the information bits arrive in serial form at the input of a memory register and are delivered at the output of this memory register, also in serial form, with a certain delay, and in the order of their input. This is why they are called "first-in-first-out" or FIFO memories. FIG. 1 shows a known embodiment of a memory of this kind. This memory comprises N series-connected register elements. These N register elements are, for example, D type flip-flop circuits, such as the flip-flop circuit 11. They are bistable and synchronous flip-flop circuits.
By the term "bistable", it is understood that the Q output of the flip-flop circuit may take two different stable logic states (the high state, and the low state). The expression "synchronous" means that the output of the flip-flop circuit can change its state only when a signal edge, for example, a leading edge, is applied to a clock input CP of the flip-flop circuit. It will then be said that the flip-flop circuit is activated. The logic state of the D input is then copied at the Q output and held until the appearance of a new edge on the input CP, irrespective of any changes undergone by the signal at the D input between these two edges.
The serial connection of the D flip-flop circuits is such that the Q output of a particular flip-flop circuit is transmitted to the D input of the next flip-flop circuit. For example, the memory register 1 of the prior art structure shown in FIG. 1 has twelve D type flip-flop circuits that are thus series-connected, each simultaneously receiving a clock signal CK at its clock input CP. The data bits that appear successively at the input E of the register provided by the D input of the first flip-flop circuit 12 are successively transmitted from one flip-flop circuit to the next at the rate of the leading edges of the clock signal. The data bits are finally delivered to the output of the last flip-flop circuit 13 after the twelfth pulse of the clock signal. The output of the flip-flop circuit 13 is the output of the memory register.
If necessary, in a manner that is also known, the output of the register is looped to its input through data routing means so as to obtain the circulation of the data in the serpentine route defined by the thus looped register. Thus, the loss of information is prevented. The register then necessarily has a number of flip-flop circuits at least equal to the number of data bits to be stored, and is preferably equal to this number.
FIG. 2 shows a schematic view of the register 1 of FIG. 1, showing the twelve elements of the register R0 to R11 provided by the D flip-flop circuits of FIG. 1. The arrows between the register elements represent the transmission of the data bits from one element to another. This transmission is done at the rate of the leading edges of the clock signal CK applied simultaneously to all the elements.
The major drawback of this known sequential access memory architecture lies in its very high dynamic consumption of current. Indeed, a full write cycle for writing an N bit word in a register that comprises N register elements requires N clock periods during which the N registers are activated simultaneously. If e denotes the elementary energy consumed by a register element owing to its activation by an edge of the clock signal (the term used will be its "dynamic consumption" as opposed to its "static consumption" during the holding phases that elapse between two successive activation operations), then a value of dynamic energy equal to ED=N.times.N.times.e is consumed by the register.
This energy consumption is detrimental for two reasons. Firstly, the energy consumed by the register gives rise to a heating of the electronic circuit that incorporates it. The heat caused needs to be dissipated. This is a factor that hampers the present development towards very large-scale integration. Secondly, this consumption of current discharges the accumulator that supplies the circuit for its operation. This is especially detrimental when the circuit is used by a portable apparatus that is self-supplied or remote-supplied by inductive coupling.